Safety assessment and maximum permissible activity concentration in the primary coolant of an experimental multipurpose high temperature gas cooled reactor

The safety and the maximum permissible activity concentration in primary coolant were evaluated for the experimental, multipurpose, high temperature gas-cooled reactor, JHTR. The maximum permissible activity concentration in primary coolant was set up, considering the following four items: 1) the public exposure dose under normal plant operation, 2) the public exposure dose under accident condition, 3) the exposure dose of the maintenance workers after plant shut down, and 4) the concentration of tritium in the primary coolant. The procedure of setting up the maximum permissible activity is shown with the flow chart. As for the analytical method, the analytical data for the transfer of fission products at the normal operating condition were at first defined as follows, the thermal output of the reactor 50 MWt, the inventory of the primary coolant gas 681 kg, the estimated fuel failure rate 0.1%, the plate out rate 40%, the filter efficiency 90% for iodine and 99% for metallic fission products and so on. Then, the analytical models of radioactivity release under the normal and accident conditions are shown. The calculation was conducted, utilizing the computer codes, FIPPIZ, RAD.MP, GASRUP, KATEC and HTCORE. The pressure drop behavior of the primary coolant system at primary pipe rupture, the subsequent temperature rise of the fuel and the pressure vessel, and the helium release rate within the auxiliary building after one tube ruptured in the intermediate heat exchanger are plotted as the analytical results. Finally, the actual values of the maximum permissible activity are presented for rare gas, iodine, solid fission products and tritium. (Nakai, Y.)